True-section extruded plastic cutter filament system

ABSTRACT

An improved cutting filament for use in a rotary trimmer is an extruded resin, being subject to vertical extrusionto sustain true-section profile, and to structural orientation to enhance the strength and toughness of the material. One embodiment, a polygonal extruded section is drawn down in size, being found to possess improved abrasive, herbiage-cutting characteristics. One embodiment has a finned profile with at least one fin in the form of a projecting rib, preferably with a scarified surface, to enhance its abrasive characteristics. In a polygonal section, the fin portions of the section are located at the apices of the polygon. The draw-down molecular orientation process enables the use of an extrusion die of sufficient size that the scarified finning provision is readily incorporated into the die. A mix of plastic resins consisting of 20 to 40 percent by weight of linear low density, or high density polyethylene, with the balance (75-80%) being polypropylene extrudes as a pliable, high strength, impact and abrasion resistant copolymer filament of adequate High Heat Resistance (HHR) to withstand use as a trimmer filament.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention is directed to extruded plastic filaments, and inparticular to a filament used as a cutting medium, in applications suchas grass trimmers, and to apparatus and process for the manufacturethereof

[0003] 2. Description of the Prior Art

[0004] The use of extruded plastic filament as a cutting agent for grasstrimmers is widespread.

[0005] In most instances the filament is mounted on a reel that issupported on an inverted rotatable spindle of a vegetation trimmingmachine. Such machines have a safety guard that at least partiallyoverlies the extended filament, such that in operation the filament cancontact vegetation such as grass, light shrubs, weeds and the like incutting reaction therewith, due to the speed of rotation of the machine.

[0006] As a consequence, the free tip of the filament, which does muchof the cutting is progressively abraded, requiring the release of acompensatory length of the filament from its reel in order to continueoperation Existing filaments are very susceptible to damage when cominginto contact with common obstacles, particularly masonry, stones,drainpipes, etc.

[0007] The plastic resin presently most used consists of a nylon or anylon/polyethylene copolymer, extruded through a die.

[0008] Such prior art filaments are subject to a high rate of breakageat the tip when impacting hard objects such as posts, cement blocks,walls, down-spouts and the like. Ribbed filament sections of the priorart, being of smooth finish, do not provide a very effective cuttingprofile.

[0009] In manufacturing such filaments, difficulties have beenexperienced in maintaining a true profile, due to the flattening effectsof gravity on horizontally oriented extrudate. With higher temperatureextrusions, the problems of diametical distortion have increased, sothat the maintenance of tolerances is most difficult. Certain prior arttrimmer filaments possess an inadequate High Heat Resistance (H.H.R.)value, so that in the high temperature environment of sustainedoperation self adhesion takes place.

SUMMARY OF THE INVENTION

[0010] The present invention provides an improved cutting filament foruse in a rotary trimmer.

[0011] In one embodiment, a polypropylene filament is extruded, andsubjected to an orientation process, to enhance the strength andtoughness of the material.

[0012] In another embodiment, a polypropylene or polypropylene blendedwith polyethylene of low or of high density, extruded as an oversizepolygonal section, is drawn down in size, being found to possessimproved abrasive, herbiage-cutting characteristics.

[0013] A further embodiment has a finned profile with at least one finin the form of a projecting rib. In the case of a polygonal section,this is preferably modified, wherein fin portions of the section arelocated at the apices of the polygon.

[0014] The application of a drawdown molecular orientation processenables the use of an extrusion die of sufficient size that the finningprovision is readily incorporated into the die. The effective cuttingaction of the fin or fins is greatly enhanced when the sides and bottomsurfaces of the die grooves are scarified, to be irregularly rough andjagged.

[0015] The present invention thus provides an extruded filament for useas an abrasion medium, wherein the filament consists of a polyolefinmixture having the molecular structure thereof at least partiallyaxially oriented, to impart enhanced strength and abrasion resistance tothe filament.

[0016] In particular, the filament is found to be useful as a cuttingmedium in a rotary herbiage trimming tool.

[0017] It has been found in one embodiment that a mix of plastic resinsconsisting of 20 to 40 percent by weight of linear low densitypolyethylene is mixed with 60-80 weight percent polypropylene copolymeror homopolymer as the balance a high strength, impact and abrasionresistant, pliable filament of adequate High Heat Resistance (HHR) isproduced, which is able to withstand use as a trimmer medium withoutself-welding, in the usual hot working environment.

[0018] As an example, a Montell polypropylene, listed as an extrusiongrade homopolymer No. 6823, and with characteristics of MFR—0.5 dg/min;Tensile Strength—4800 psi; Flexural modulus—180,000 psi; IZOD Impact—1.5ft-lb/in; HDT—205 F. degrees; rated Underwriters Lab 94HB, and with FDAY1 rating gave excellent working results when extruded and processed inaccordance with the presently disclosed process, with a service lifeimprovement as a trimmer medium of more than 40 percent over standardcommercial nylon trimmer filament.

[0019] In another exaMPLE, Montell's SV-152 (nucleated) super highimpact copolymer is used.

[0020] Some of these copolymers have a low Melt Flow Rate (MFR) of 2 orless, requiring extrusion at 500F. degrees and higher; the lower the MFRvalue, the higher is the required extrusion temperature.

[0021] The combining of polyethylene resin, of high or low density tothe polypropylene improves pliability. The polypropylene supplied mayincorporate high density polyethylene as one of its constituents.

[0022] One embodiment of the present filament has a polygonal crosssection, wherein the apices of the polygon appear to serve as cuttingedges, when employed with a rotary tool as an abrading agent.

[0023] A further embodiment of the filament may have a finned profile,to enhance the abrading action of the filament.

[0024] In the case of a filament having a polygonal profile, it may haveat least one fin portion extending from an apex of the polygonalprofile.

[0025] The finned filament may have a number of the fins, as in the caseof a polygonal section, having each fin extending individually from arespective apex of the polygonal profile.

[0026] The provision of a scarified finish to the working surface of afilament has been found to greatly enhance the abrading effectiveness ofthe filament. This applies to plain, ribbed and circular sectionfilaments.

[0027] In view of the drawing-down of the extruded section, a die ofsufficient size is used wherein a scarified finish may be applied tosurface portions of the die, using a toothed engraving tool.

[0028] The process of providing an oriented filament in accordance withthe present invention includes the steps of: providing a die ofpredetermined size, form and finish; extruding a selected plastic resinat a first, extrusion temperature in a predetermined first temperaturerange; minimally cooling the extrudate to a first handling temperature;passing the extrudate through a first drawing apparatus operating at afirst linear speed; heating the extrudate to a second, orientingtemperature in a predetermined second temperature range; drawing-down toa filament the re-heated extrudate at a second linear speed in the rangesix to nine times the first linear speed, to orient the structure of thefilament in a substantially axial direction by tensioning by means of ahigh speed orienting puller, including the step of cooling the orientedfilament prior to passage thereof through the orienting puller; andcoiling the oriented filament on a spool winder. In the case of a resinhaving a high percentage content of polypropylene, a normal cooling bathmay be somewhat inadequate, due to the high extrusion temperaturerequired for this material. It has been found that a venturi cooler maybe employed, having a conical shape, through which the extrudate ispassed. Coolant admitted at the throat flows about the extrudate ineffective cooling relation as it passes through the diverging shape ofthe conical cooler., usually into a cooling bath.

[0029] In the case of a circular section filament, in order to maintaina substantially circular extruded section, within reasonably specifiedtolerances, and to avoid the section flattening that can readily occurwith high temperature extrusion, the extrusion die is preferablyoriented with its polar axis vertical, discharging substantiallydirectly into a coling bath

[0030] In an extrusion head having an annular array of die orifices, adispersaL cone is located upstream of the die orifices, to facilitateuniformity of product mix.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Certain embodiments of the invention are described by way ofillustration, without limitation thereto other than as set forth in theaccompanying claims, reference being made to the accompanying drawings,wherein:

[0032]FIG. 1 shows a schematic arrangement of a portion ofhexagon-section, polygonal filament according to the present invention;

[0033]FIG. 2 is a schematic cross-section of a die for extruding ascarified ribbed filament in accordance with the invention;

[0034]FIG. 3 is a diagramatic representation of a subject filamentextrusion and orientation process;

[0035]FIG. 4 is an enlarged portion of FIG. 3, showing the venturicooler;

[0036]FIGS. 5, 5A and 6 are cross-sections of hexagonal and circularfilaments, some being shown having scarified finishes on the workingsurfaces thereof,

[0037]FIG. 7 is a front elevation, shown in diametrical section, of amulti-orifice vertical extrusion head;

[0038]FIG. 8 is a side elevation in diametrical section of the FIG. 7embodiment (to a different scale); and,

[0039]FIG. 9 is a view of an extrusion die face having an array ofcircular dies.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Referring to FIG. 1, a length 10 of a polygonal filament (not toscale) is shown, a hexagonal section being shown for purposes ofillustration.

[0041] In FIG. 2 a hexagonal die block 12 has a hexagonal die face 14,each of the apices of which is relieved by a recess 16. In an actual dieembodiment the dimension “A” is 0.300 inches; dimension “B” is 0 250”.The recesses 16 are made by an undersized hacksaw blade, which scarifiesthe effective working sufaces. The effects of the scarified die surfacesare shown in the product 50, at FIG. 5. FIG. 6 shows a scarifiedcylindrical product 52, made by a scarified circular die (not shown).FIG. 5A illustrates the true circular profile of extrudate achieved by avertical extruder, by the avoidance of gravitational flattening of thehot extrudate.

[0042]FIG. 3 shows an extrusion and orientation line 18 for producingoriented filament.

[0043] The line 18 has a vertical extruder 20 feeding to a serialarrangement of a hot water cooling tray 22, a take-off puller 24equipped with soft rubber belts (not shown), a re-heater hot air oven26, a cooling tray 28, a high speed orienting puller 30, and a spoolwinder 32. The cooling tray 22 may have a recirculation pump 25, withtemperature adjuster 27 for heating or cooling the water.

[0044] Referring to FIG. 4, a venturi cooler 22 is shown havingdivergent conical walls 40, with a coolant gallery 42 about the throatportion 44. Hot water is supplied by way of supply pipe 46, as a showerover the extrudate as it passes through the cooler 22. High rates ofcooling may be provided by evaporative cooling, if required.

[0045] In one example, a resin blend, comprising 20 to 40 weight %linear low density polyethylene with the balance being polypropylenecopolymer or homopolymer was extruded, the extruder 20 operating in therange 450 to 500 F. degrees. The extrudate 21 passing through the cooler20 to the water cooling tray 22, operating at 200 F. degrees; fromthence to the puller 24, having soft rubber belts to grip withoutmarring the extruded profile From the puller 24 the extrudate 21 isdrawn through the hot air oven 26, operating in the range 425 to 475 F.degrees, which heats the extrudate sufficiently to permit extremeextension, with consequent orientation. This extension is generated bythe orienting puller 30, having pinch rolls operating at a speed in therange six to nine times faster than the take-off puller 24.

[0046] A cooling water tray upstream of the puller 30 is operated in therange 80 to 100 F. degrees.

[0047] From the puller 30, the reduced, oriented, or partially orientedfilament product is wound onto spools by spool winder 32.

[0048] Referring to FIGS. 7, 8 and 9, an injection die 50 has across-head portion 52 for attachment to an injection molding machine,not shown. A downwardly extending die head 54 has a divergent passage 56containing a dispersion cone 58, that forms in conjunction with thepassage 56 an annular arrangement of convergent flow passages, eachpassage containing a die such as the die 12, in accordance with thepresent invention.

1. An extruded filament for use as an abrading medium in a rotary device, wherein said filament consists of plastic having the molecular structure thereof substantially axially aligned, to impart enhanced strength and abrasion resistance to the filament.
 2. The filament as set forth in claim 1, wherein said filament has a polygonal cross section.
 3. The filament as set forth in claim 1, wherein said filament has a finned profile.
 4. The filament as set forth in claim 3, wherein said filament has a polygonal profile, with at least one fin portion extending from an apex of the polygonal profile.
 5. The filament as set forth in claim 4, having a plurality of said fins, each extending individually from a respective said apex.
 6. The filament as set forth in claim 1 wherein said plastic consists of a copolymer including 20 to 40 weight percent of linear low density polyethylene mixed with 60 to 80 weight percent of polypropylene.
 7. The filament as set forth in claim 6, wherein said filament has a polygonal cross section.
 8. The filament as set forth in claim 6, wherein said filament has a finned profile.
 9. The filament as set forth in claim 8, wherein said filament has a polygonal profile, with at least one fin portion extending from an apex of the polygonal profile.
 10. The filament as set forth in claim 9, having a plurality of said fins, each extending individually from a respective said apex.
 11. The filamant as set forth in claim 1, having an abrading working surface, wherein said suface is scarified.
 12. The filamant as set forth in claim 2, having an abrading working surface, wherein said suface is scarified.
 13. The filamant as set forth in claim 5, having an abrading working surface, wherein said suface is scarified.
 14. An extrusion die having a polygonal cross section, at least one apex, at the juncture of two sides of said polygonal section having an outwardly extending recess, in use to form a rib on the outer surface of a filament extruded through said die, at least a portion of the surface of said recess being scarified.
 15. The die as set forth in claim 14, having a plurality of pairs of faces in mutually adjacent relation to form a corresponding plurality of apex, each apex thereof having a said recess, in use to form a polygonal extrudate having an outwardly extending rib located between each pair of adjacent faces.
 16. The process of making an abrasion-resistant filament consisting of the steps of forcing heated plastic under pressure through a die of predetermined cross-sectional size and shape to form a filament extrudate; cooling said extrudate; gripping said cooled extrudate; drawing said cooled extrudate away from said die at a predetermined first rate of draw, to facilitate the flow of said extrudate from the die; heating the extrudate to an orienting temperature within a predetermined temperature range, and drawing said filament at a second rate greatly in excess of said first rate, to draw down the profile of and axially orient said filament, whereby the strength and abrasion resistance of said filament is enhanced
 17. The process as set forth in claim 16, wherein said extrudate is extruded through said die at a temperature in the range 450 to 500 F. degrees; and wherein said predetermined orienting temperature range is 425 to 475 F. degrees
 18. The process as set forth in claim 16, including cooling said filament subsequent to said draw down, and winding the cooled filament on a spool. 